Steel concrete and composite design of tall bulding - bungale s taranath

Full and partial loading 2.5.6.5 Gust effect factor 1 Rigid structures—simplified method 2 Rigid structures—complete analysis 3 Flexible or dynamically sensitive buildings 2.5.6.6 Calcu

Khing Cigrg Coin

Steel, Concrete, and Composite Design

of Tall Buildings

Bungale S Taranath

Second Edition

McGraw-Hill New York San Francisco Washington, D.C Auckland Bogota

Caracas Lisbon London Madrid Mexico City Milan

Montreal New Delhi San Juan Singapore

Sydney Tokyo Toronto

1 Building, Iron and steel 2 Concrete construction

3 Composite construction 4 Tall buildings—Design and

construction 5 Structural engineering

TH1611.T37 1997

693'.71-—dc21 96-49612

CIP

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Daughter Anupama and Son Abhiman

Contents

Preface xix Acknowledgements xxi

Chapter 1 General Considerations 1

1.3 Structural vocabulary: Case Studies 9

1.3.4 Bank of China Tower, Hong Kong 15

1.3.9 AT&T Building, New York City 31

» 1.3.12 Jin Mao Tower, Shanghai, China 35

1.3.15 Leaning Tower, a building in Madrid, Spain 44

, 1.3.26 First Interstate World Center, Los Angeles 75

vii

2.3 Extreme Wind Conditions

introduction 2.3.2 Thunderstorms

Introduction

Variation of wind velocity with height

Turbulent nature of wind Probabilistic approach to wind load determination Vortex-shedding phenomenon

Dynamic nature of wind

Cladding pressures

2.4.7.1 Introduction 2.4.7.2 Distribution of pressures and suctions

2.4.7.3 Local cladding loads and overall design loads

’ 2.5 Code Wind Loads

introduction BOCA National Building Code (1996) Standard Building Code (1991) Uniform Building Code (UBC 1994)

2.5.4.3 Special wind regions 2.5.4.4 Hurricanes and Tornadoes

2.5.4.5 Exposure effects 2.5.4.6 Site exposure 2.5.4.7 Design wind pressures

2.5.4.8 The C, factor 2.5.4.9 Pressure coefficient, C,

2.5.4.10 Importance factors /,, 2.5.4:11 Design exampie, UBC 1994

ANSI/ASCE 7-93

2.5.5.1 Overview 2.5.5.2 Design pressures for the main wind-force resist-

ing system 2.5.5.3 Design wind pressure on components and

cladding 2.5.5.4 External pressure coefficient c,

2.5.5.5 internal pressure coefficient c,;

Contents

(2) Non-flexible structure; Height >500 ft

(3) 60-storey flexible building

(4) 50-storey flexible building 2.5.5.7 — Sensitivity study of gust response factor G 2.5.6 ASCE 7-95: Wind load provisions

2.5.6.1 Introduction 2.5.6.2 Overview 2.5.6.3 Wind speed-up over hills and escarpments: Kz

factor 2.5.6.4 Full and partial loading 2.5.6.5 Gust effect factor

(1) Rigid structures—simplified method (2) Rigid structures—complete analysis (3) Flexible or dynamically sensitive buildings 2.5.6.6 Calculation of gust effect factor: design example 2.5.6.7 Sensitivity study of gust response factor 2.5.6.8 Calculation of wind pressures: design example 2.5.7 National Building Code of Canada (NBC 1990)

2.5.7.1 Simple procedure

(a) Reference pressure, q_

(b) Exposure factor, C, (c) Gust effect factor, (dynamic response factor)

C, (d) Pressure coefficient, C,

2.5.7.2 Experimental procedure

2.5.7.3 Detailed procedure:

(a) Exposure factor, C, (detailed procedure) (b) Gust effect factor, C, (detailed procedure) 2.5.7.4 Design example

2.5.7.5 Wind-induced building motion 2.6 Wind Tunnel Engineering

2.6.1 Introduction 2.6.2 Description of wind tunnels

2.6.3 Objective of wind tunnel tests

2.6.4 Rigid model studies 2.6.5 Aeroelastic study

2.6.5.1 Model requirments 2.6.6 High-frequency force balance model 2.6.7 Pedestrian wind studies

2.7 Field Measurements of Wind Loads 2.8 Motion Perception: Human Response to Building Motions 2.9 Comparison of Code and Wind Tunnel Test Results 2.10 Chapter Summary

Chapter 3 Seismic Design

3.1 Introduction 3.1.1 Nature of earthquakes 3.1.2 Some recent earthquakes

3.2 Tall Building Behavior During Earthquakes

3.2.1 introduction

3.2.2 Response of tall buildings

3.2.3 Influence of soil 3.2.4 Damping

3.2.5 Building motion and deflections 3.2.6 Seismic separation

3.3 Seismic Design Concept

3.3.1 Determination of forces

3.3.2 Design of the structure 3.3.3 Structural response 3.3.4 Path of forces 3.3.5 Demands of earthquake motion 3.3.6 Response of buildings

3.3.7 Response of elements attached to buildings

3.3.8 Techniques of seismic design

3.3.8.1 Layout

3.3.8.2 Structural symmetry

3.3.8.3 trregular buildings 3.3.8.4 Lateral force-resisting systems

3.3.8.5 Diaphragms 3.3.8.6 Ductility 3.3.8.7 Nonstructural participation

3.3.8.8 Foundations

" 3.3.8.9 Damage control features

3.3.8.10 Redundancy 3.4 1994 UBC Equivalent Lateral Force Procedure (Static Method)

3.4.1 Design base shear 3.4.2 Base shear distribution along building height

3.4.3 Horizontal distribution 3.4.4 Torsion

3.4.5 Story shear and overturning moments 3.4.6 Discontinuity in lateral force-resisting elements 3.4.7 p-delta effecis

3.4.8 Continuous load path

3.4.9 Redundancy 3.4.10 Configuration 3.4.11 Design example

3.5 Dynamic Analysis Procedure

3.5.1 Introduction 3.5.2 Response spectrum method 3.5.3 Development of design response spectrum 3.5.4 Time-history analysis

3.6 Seismic Vulnerability Study and Retrofit Design

3.6.1 Introduction

3.6.2 Code sponsored design

3.6.3 Alternative design philosophy

3.6.3.1 FEMA 178 method 3.6.3.2 Tri-services manual

34633 SEAOCs Vision 2000: performance-based

engineering

3.7 Dynamic Analysis: Theory 3.7.1 Introduction 3.7.2 Systems with single-degree-of-freedom 3.7.3 Multi-degree-of-freedom systems 3.7.4 Modal superposition method

3.7.4.1 Normal coordinates

3.7.4.2 Onhogonality 3.8 Summary

Chapter 4 Lateral Systems: Steel Buildings

4.1 Introduction 4.1.1 Steel in high-rise buildings 4.2 Frames with Semi-Rigid-Connections 4.2.1 Introduction

4.2.2 Review of connection behavior 4.2.3 Beam line concept

4.2.4 Type 2 wind connections

4.2.4.1 Design outline for type 2 wind connections

4.3.4 Calculation of drift

4.4 Braced Frames 4.4.1 Introduction 4.4.2 Behavior

4.4.3 Types of braces

4.5 Staggered Truss System 4.5.1 Introduction 4.5.2 Physical behavior

4.9.3 Shear-lag phenomenon 471

4.12 Ultimate High-Efficiency Structures 495

Chapter 5 Lateral Bracing Systems for Concrete Buildings 501

5.4 Coupled Shear Walls 506

5.7 Rigid Frame with Haunch Girders 508

Contents

5.11 Exterior Diagonal Tube 5.12 Modular or Bundled Tube 5.13 Miscellaneous Systems

Chapter 6 Lateral Systems for Composite Construction

6.1 Introduction

6.2 Composite Elements

6.2.1 Composite slabs

6.2.2 Composite girders 6.2.3 Composite columns 6.2.4 Composite diagonals

6.2.5 Composite shear walls 6.3 Composite Building Systems 6.3.1 Shear wall systems 6.3.2 Shear wall-frame interacting systems

6.3.3 Tube systems

6.3.4 Vertically mixed systems 6.3.5 Mega frames with ‘super columns 6.4 Example Projects

6.4.1 Composite steel pipe columns

6.4.1.1 Pacific First Center

6.4.1.2 Fremont Experience, Las Vegas

6.4.2 Formed composite columns

6.4.2.1 Interfirst Plaza, Dallas

6.4.2.2 Bank of China Tower, Hong Kong 6.4.2.3 The Bank of South West Tower, Houston, Texas 6.4.3 Composite shear walls and frames

6.4.3.1 First City Tower, Houston, Texas 6.4.4 Composite tube system

6.4.4.1 The America Tower, Houston, Texas

6.4.5 Conventional concrete system with partial steel floor framing

6.4.5.1 The Huntington, Houston, Texas 6.5 High-Efficiency Structure: Structural Concept

Chapter 7 Gravity Systems for Steel Buildings

7.1 Introduction 7.2 Design Loads 7.3 Metal Deck Systems 7.4 Open-Web Joist Systems

7.5 Wide-Flange Beams

7.5.1 Bending 7.5.2 Shear 7.5.3 Defleclions

8.1.6 Band beam system 8.1.7 Haunch girder and joist system

8.1.8 Beam and slab system 8.1.9 Design examples

8.1.9.1 One-way slab-and-beams system

8.1.9.2 T-beam design

8.1.9.3 Analysis of two-way slabs

8.2 Prestressed Concrete Systems

8.2.1 Method of prestressing 8.2.2 Materials

8.2.7.4 Continuous spans

Example 1 Example 2 Example 3

End bay design , 8.2.7.5 Mild steel reinforcement design (strength design

for flexure)

Chapter 9 Composite Gravity Systems

9.1 Composite Metal Decks 9.1.1 General considerations 9.1.2 SDI specifications 9.2 Composite Beams

9.2.1 General considerations

9.2.2 AISC design specifications

Example 9.3 Composite Haunch Girders

9.4 Composite Trusses

9.5 Composite Stub Girders 9.5.1 General considerations 9.5.2 Behavior and analysis

Chapter 10 Analysis Techniques

Preliminary Hand Calculations 10.1

Portal method Cantilever method Lateral stiffness of frames Framed tube structures Coupled shear walls Lumping Techniques, Partial Computer Models

Torsion

10.4.1 10.4.2 10.4.3 10.4.4 10.4.5 10.4.6 10.4.7 10.4.8 10.4.9 10.4.10 10.4.11 10.4.12 10.4.13

Introduction Concept of warping behavior: |-section core

Sectorial coordinate w

Shear center Principal sectorial*coordinate w diagram Sectorial moment of inertia /,,

Shear torsion constant J Calculation of sectorial properties: worked example General theory of warping torsion

Torsion analysis of shear wail structures: worked examples Torsion analysis of steel braced core: worked example Warping torsion constants for open sections

Computer analysis Modeling techniques Warping stiffness of floor slab Finite element analysis Twisting and warping stiffness of open sections Stiffness method using warping-column model

11 Structural Design

Steel Design

11.4.1 41.1.2 14.1.3

sections

11.1.3.3 Allowable bending stresses 11.1.3.4 Allowable shear stress Members subjected to compression

11.1.4.1 Buckling of colurnns 11.1.4.2 Column curves

xvi Contents

11.1.5

11.1.6 11.1.7 11.1.8 11.1.9

11.1.4.4 Stability of frames: effective length concept Members subjected combined axial load and bending

11.1.5.1 Secondary bending: P-A effects

11.1.5.2 Interaction equations 11.1.5.3 Direct analysis for P-A effects

Calculation of stress ratios

Design of continuity plates

Design of doubler plates

Additional seismic requirements (UBC 1994) 11.1.9.1 Ordinary moment frames 11.1.9.2 Special moment-resisting frames 11.1.9.3 Braced frames

11.1.9.4 Eccentrically bracedÄrames (EBF)

11.1.9.5 Special concentrically braced frames 11.2 Concrete design

41.2.1 11.2.2

11.2.2.1 Design for flexture

11.2.2.2 Design for shear

Shear reinforcement in coupling beams:

seismic design

11.2.2.3 Joint design of special moment-resisting frames

Determination of pane! zone shear force

Determination of effective area of joint

Panel zone shear stress

11.2.2.4 Beam column flexural capacity ratios

Column design

11.2.3.1 Generation of bi-axial interaction surfaces 11.2.3.2 Determination of moment magnification factors 11.2.3.3 Additional seismic requirements

11.2.3.4 Design for shear

Determination of factored forces Determination of concrete shear capacity Determination of shear reinforcement Shear wall design

11.2.4.1 _ Design for overturning moment and axial load

11.2.4.2 Design for shear 11.2.4.3 Additional seismic requirements 11.2.4.4 Load-moment interaction diagram Comments on seismic details

Joint shear

Why strong column—weak beam Why minimum positive reinforcement

Chapter 12 Special Topics

12.1 Differential Shortening of Columns

Mat for an 85-storey building ` 937

12.8 Seismic Design of Diaphragms 938

12.8.2 Diaphragm behavior 940

12.9 Earthquake Hazard Mitigation Technology 949

12.11.3 Structural steel quantities 976

Appendix A: Conversion Factors: U.S Customary Units to SI Metric Units 991

Preface

This book has been developed to serve as a comprehensive reference for designers of tall building structures Structural design aspects of steel, concrete and composite buildings with particular reference to wind and seismic loads are discussed Methods of providing gravity and lateral load resistance including the state-of- the-art systems are discussed as well as the many facets of designing

of structural elements

This is itended as a practical book useful to engineerig students, consulting engineers, architects, engineers employed by federal, state and local governments and educators The material has been pre- sented in easy-to-understand form to make it useful to young engineers with their first high-rise, and to offer new approaches to those who have been involved with tall building structures in the

past Numerous examples illustrating design procedures are worked

out in detail

The book begins with a description of structural systems of tall buildings built around the world within the past two decades The purpose is to familiarize readers with the information that currently resides in these designs, for the engineering mind constantly needs

past solutions and tried formats as anchors before it can break new ground or differ markedly from conventional wisdom

Chapter 2 deals with different approaches for evaluating wind loads appropriate for building design Building code and wind tunnel procedures are discussed, including analytical methods for determin- ing building response related to occupant comfort

Chapter 3 outlies seismic design, highlighting the dynamic be- havior of builidings Static, dynamic, and time-history analyses are

described Seismic vulnerability study and retrofit design of buildings

not meeting current building code detailing standards are also

discussed

The design of framing systems for lateral forces is the subject of the fourth, fifth and sixth chapters Traditional and newer-type bracing systems in steel, concrete, and combinations of the two, called composite construction, are analyzed

xix

XX Preface

The seventh, eighth, and ninth chapters are dedicated to gravity design of vertical and horizintal building systems In addition to : common systems, novel techniques such as composite stub girders are also discussed

Chapter 10 focuses on the analysis of structural systems and components Approximate methods are discussed first, followed by computer modeling techniques for two and _ three-dimensional analyses Torsion analysis, including the author’s original Ph.D work on warping behavior of open-section shear walls, is covered in detail This information is particularly useful in making preliminary designs and verifying three-dimensional computer models

Chapter 11 gives an overview of code check process for verifying the design of structural elements Design methods and equations from leading specifications including special requirements for seismic design are presented for ready reference

The concluding chapter is devoted to the discussion of various topics unique to the design of tall buildings Differential shortening of

columns, design of curtain walls, mechanical damping systems for reducing wind-induced sway accelerations, drilled pier and mat

foundations, earthquake mitigation technologies are some of the subjects covered in this chapter Brittle fracture of welded moment connections subjected to large inelastic demands is discussed Unit structural quantities for estimating prelimiary steel tonnage of high rise steel and composite buildings are described, followed by an update of the 1997 Uniform Building Code which is expected to serve

as a source document for the entire United States begining in the year 2000

The book attempts to achieve a number of objectives; it is intended

to bridge the gap between a novice and an experienced designer while serving simultaneously as a comprehensive resource document The first and foremost audience is the practicing structural engineer ranging from the young that have just now entered the profession to those with considerable experience The scope of the book is inten-

tionally broad with enough in-depth material to make it useful for

practitioners of structural engineering in all stages of their careers It

is hoped that this book will also serve as a teaching tool for advanced structural courses in colleges and universities

Acknowledgements

The author wishes té express his sincere appreciation and thanks

to many individuals and friends who helped in this endeavour: John

A Martin Sr and Trailer A Martin Jr., for their support and encouragement; Mysore V Ravindra, Dr P V Banavalkar, Dr Walter P Moore Jr., and John L Tanner for providing information

on buildings designed by their firms; Dr Roger M De Julio Jr., for

comprehensive review of Chapter 3; Dr Farzad Naeim, Brett W

Beekman, and Kalman V Benuska for reviewing parts of the

manuscript and offering many helpful suggestions; Mike Baltay for

his assistance in gust-sensitivity study; Lupe Infante, Rima Roerish, Ivy Policar, and Betty Cooper for typing parts of the manuscript; Evita Oseguera and Andrew Besirof for supplying project photo- graphs; and Emilio Rodriguez, Raul Oseguera, Mike Mittelstaedt,

and Ben Kirton for their general assistance

Dr Madhu B Kanchi offered many helpful suggestions in the preparation of the manuscript To this I am grateful to him I thank

the entire engineering staff of John A Martin & Associates for

sharing information about their projects Thanks to Jack Martin Sr and Trailer Martin Jr., for giving permission to include descriptions

of their projects in this book

My daughter Anupama and son Abhiman provided a great deal of support and help to this project My sincere thanks to both of them, especially my son who typed most of the manuscript

Most deserving of special recognition is my wife Saroja My source

of inspiration, she helped in all phases of this endeavor—from the manuscript’s inception to its final submission Her “sunny disposi- tion” transformed the ardous tasks of manuscript preparation into a fun-filled family project My deepest appreciation and thanks for her

unconditional love, support, sacrifice and help Without her patience,

this book could not have been written

sheer audacity in their vertical scale may often give them the dubious title of monuments The difference in the usage of buildings, from

solid monumental structures to space enclosures, in itself has not changed the basic stability and strength requirements; the structural issues are still the same, the materials and methods are different

In the design of early monuments, consideration of spatial interac-

tion between structural subsystems was relatively unimportant,

because their massiveness provided for strength and stability In comparison, the size and density of structural elements of a contem- porary tall building are strikingly less, and continue to diminish motivated by the real-estate market, aesthetic principles and innova- tive structural solutions Thus the trend in high-rise technology can

be thought of as a progressive reduction in the quantity of structural material used to create the exterior architectural enclosure and the

spaces within

To be successful, a tall building must economically satisfy the often

conflicting demands imposed by various trades such as mechanical,

electrical, structural and architectural In doing so, from a structural point of view, a building can be defined tall when its height creates different conditions in the design, construction, and use than the

1

2 Chapter One

conditions that exist for its lower brethren These conditions are _manifest when the effects of lateral loads begin to influence its design For example, in the design of tall buildings, in addition to the requirements of strength, stiffness and stability, the lateral deflec- tions due to wind or seismic loads should be controlled to prevent both structural and nonstructural damage Also the wind response of top floors in terms of their accelerations during frequent wind storms, should be kept within acceptable limits to minimize motion percep- tion and discomfort to building occupants

The trend in high-rise architecture is to create an overall spatial form with an intricate detailing of cladding system (Fig 1.1) The reason is uniquely to define a tower within an urban environment, and at the same time, provide interior spaces that are highly desirable to the building tenants More often, the resulting structural solution is complex However, the engineer, who until the early 1970s exercised considerable influence on the building’s architectural shape, no longer deems it necessary to do so Instead, with the

Figure 1.1 Spatial form of mod- ern high-rise architecture

Fox Plaza, Los Angeles John-

son, Fain and Pereira; Archi- tects John A Martin & Asso

Inc., Structural Engineers

General Considerations 3

immense analytical backup provided by computers, the structural

engineer has freed the architect of structural restraints, especially in

seismically benign regions Needless to say, free-form architecture

has demanded closer scrutiny of proven systems, challenging the

engineer to either modify the proven systems or to come up with new

structural solutions altogether Although it is possible to arrive at a

number of structural solutions which are equally applicable to a

particular high-rise building, the final scheme more often depends on

how best it meets other nonstructural requirements Optimization of

structural systems is thus a task that is studied in concert with other

blowing against the building or due to the inertia forces induced by ground shaking, tends both to snap it (shear), and push it over (bending) Therefore, the building must have a system to resist shear

as well as bending In resisting shear forces, the building must not

break by shearing off (Fig 1.3a), and must not strain beyond the limit of elastic recovery (Fig 1.3b) Similarly, the system resisting

the bending must satisfy three needs (Fig 1.4) The building must not overturn from the combined forces of gravity and lateral loads due to wind or seismic effects; it must not break by premature failure

of columns either by crushing or by excessive tensile forces; its

bending deflection should not exceed the limit of elastic recovery In

addition, a building in seismically active regions must be able to resist realistic earthquake forces without losing its vertical load-

carrying capacity

In the structure’s resistance to bending and shear, a tug-of-war ensues that sets the building in motion, thus creating a third engineering problem; motion perception or vibration If the building sways too much, human comfort is sacrificed, or more importantly,

non-structural elements may break resulting in expensive damage to the building contents and causing danger to the pedestrians

A perfect structural form to resist the effects of bending, shear and

excessive vibration is a system possessing vertical continuity ideally

located at the farthest extremity from the geometric center of the

building A concrete chimney is perhaps an ideal, if not an inspiring

engineering model for a rational super-tall structural form The

Figure 1.3 Building shear resis-

tance: (a) building must not break; (b) building must not deflect excessively in shear

General Considerations 5

quest for the best solution lies in translating the ideal form of the chimney into a more practical skeletal structure

With the proviso that a tall building is a beam cantilevering from

earth, it is evident that all columns should be at the edges of the

plan Thus the plan shown in Fig 1.5(b) would be preferred over the plan in Fig 1.5a Since this arrangement is not always possible, it is

of interest to study how the resistance to bending is affected by the arrangement of columns in plan We will use two parameters, Bending Rigidity Index BRI and Shear Rigidity Index SRI, first published in Progressive Architecture, to explain the efficiency of structural systems

The ultimate possible bending efficiency would be manifest in a square building which concentrates all the building columns into four corner columns as shown in Fig 1.6a Since this plan has maximum efficiency it is assigned the ideal Bending Rigitidy Index (BRI) of 100

The BRI is the total moment of inertia of all the building columns

about the centroidal axes participating as an integrated system

The traditional tall building of the past, such as the Empire State

Building, used all columns as part of the lateral resisting system For

columns arranged with regular bays, the BRI is 33 (Fig 1.6b)

A modern tall building of the 1980s and 90s has closely spaced exterior columns and long clear spans to the elevator core in an arrangement called a “tube.” If only the perimeter columns are used

to resist the lateral loads, the BRI is 33 An example of this plan type

is the World Trade Center in New York City (Fig 1.6c)

The Sear Towers in Chicago uses all its columns as part of the lateral system in a configuration called a “bundled tube.” It also has a BRI of 33 (Fig 1.6d)

The Citicorp Tower (Fig 1.6e), uses all of its columns as part of its

6 Chapter One

lateral system, but because columns could not be placed -in the

corners, its BRI is reduced to 31 If the columns were moved to the

corners, the BRI would be increased to 56 (Fig 1.6f) Because there

are eight columns in the core supporting the loads, the BRI falls

short of 100

The plan of Bank of Southwest Tower, a proposed tall building in

Houston, Texas, approaches the realistic ideal for bending ridigity

with a BRI of 63 (Fig 1.6g) The corner columns are split and

displaced from the corners to allow generous views from office

interiors

In order for the columns to work as elements of an integrated

system, it is necessary to interconnect them with an effective

shear-resisting system Let us look at some of the possible solutions

and their relative Shear Rigidity Index (SRI)

The ideal shear system is a plate or wall without openings which

has an ultimate Shear Rigidity Index (SRD of 100 (Fig 1.7a) The

second-best shear system is a diagonal web system at 45 degree

angles which has an SRI of 62.5 (Fig 1.7b) A more typical bracing

Figure 1.5 Building plan forms: (a) uniform distribution of columns; (b)

columns concentrated at the edges

General Considerations 7

material is shown in Fig 1.7c Its SRI depends on the slope of the diagonals and has a value of 31.3 for the most usual brace angle of 45 degrees

(d) Sears Towers, BRI = 33; (e) City Corp Tower: BRI = 33; (f) building with corner and core columns, BRI = 56; (g) Bank of Southwest Tower,

RI = 63.

8 Chapter One

The most common shear systems are rigidly joined frames as shown in Figs 1.7d—g The efficiency of a frame as measured by its SRI depends on the proportions of members’ lengths and depths A frame, with closely spaced columns, like those shown in Fig 1.7e-g,

Figure 1.7 Tall building shear systems: (a) shear wall system; (b) diagonal

web system; (c) web system with diagonals and horizontals

General Considerations 9

used in all four faces of a square building has a high shear rigidity and doubles up as an efficient bending configuration The resulting configuration is called a “tube” and is the basis of innumerable tall buildings including the world’s two most famous buildings, the Sears Tower and the World Trade Center

In designing the lateral bracing system for buildings it is important

to distinguish between a “wind design” and “seismic design” The building must be designed for horizontal forces generated by wind or seismic loads, whichever is greater, as prescribed by the building code or site-specific study accepted by the Building Official

However, since the actual seismic forces, when they occur, are likely to be significantly larger than code-prescribed forces, seismic design requires material limitations and detailing require-

ments in addition to strength requirements Therefore, for build- ings in high-seismic zones, even when wind forces govern the design, the detailing and proportioning requirements of seismic

resistance must also be satisfied The requirements get progressively more stringent as the zone factor for seismic risk gets progressively higher

1.3 Structural Vocabulary; Case Studies

Having noted that a building must have systems to resist both

bending and shear, let us visit some of the world’s tall buildings

to explore how prominent engineers have exploited the concept

of SRI and BRI in their designs In describing the designs, an

attempt is made to present the structural scheme descriptions in

a doctored form This serves the educational purposes of this

book more effectively than a prosaic recounting of the design

-10 Chapter One

data Although some examples include run-of-the-mill designs that

a large number of engineers have to solve on a day-to-day basis, other

even daring in their engineering solutions Many are examples of buildings constructed or proposed in seismically benign regions requiring careful examination of their ductile behaviour and re- serve strength capacity before they are applied to seismically active

regions

‘The main purpose of this section is to introduce the reader to the existing and new vocabulary of structural systems normally con- sidered in the design of tall buildings Structural design is in a period

of mixing and perfecting structural systems such as megaframes, interior and exterior super diagonally braced frames, spine struc- tures, etc., to name a few The case studies included in this section illuminate those aspects of conceptualization and judgment that are timeless constants of the design process and can be as important and valuable for understanding structural design as are the latest computer software The case histories are based on information contained in various technical publications and periodicals Frequent use is made of personal information obtained from the structural engineers-of-record

We start our world tour in New York City to pay homage to the Empire State Building which was the tallest building in the world for more than 40 years, from the day of its completion in 1931 until 1972 when the Twin Towers of the New York’s World Trade Center exceeded its 1280 ft (381 m) height by almost 120 ft (37 m) (Fig 1.7h) The structural steel frame with riveted joints, while encased in cinder concrete, was designed to carry 100% of gravity and 100% wind load imposed on the building The encasement, although neglected in strength analysis, stiffened the frame particularly against wind load Measured frequencies on the completed frame have estimated the actual stiffness at 4.8 times the stiffness of the bare frame

1.3.1 The Museum Tower, Los Angeles

The Museum Tower, a 22-story residential complex, shown in Fig 1.8a, is part of the California Plaza complex which is one of the largest urban revitalization projects in a zone of high seismic activity

in North America The structural system for the building, located in downtown Los Angeles, consists of a tubular ductile concrete frame with perimeter columns spaced at 13 ft (3.96 m) centers intercon- nected with upturned spandrel beams (Fig 1.8b) The exterior frame

is of exposed painted concrete

General Considerations 11

The gravity system for the typical floor consists of an 8 in (203 mm)

thick post-tensioned flat plate with banded and uniform tendons running in the short and long directions of the building respectively

as shown in Fig 1.8a

Although the building is regular both in plan and elevation, and

is less than 240 ft (73 m) in height, because of transfers at the base (Fig 1.8b), a dynamic analysis using site specific spectrum was used

in the seismic design The dynamic base shear was scaled-down to a value corresponding to the 1992 UBC static base shear To preserve the dynamic characteristics of the building, the spectral accelerations were scaled down without altering the story masses The structural

design is by John A Martin & Associates, Inc., Los Angeles The architecture is by Fujikawa Johnson Asso Inc., and Barton Myers Asso Inc

Figure 1.7 (Continued) (h) Empire State Building bracing system; riveted structural steel frame encased in cinder concrete

Street level

(h)

12 Chapter One

1.3.2 Bank One Center, Indianapolis -

Bank One Center in Indianapolis is a 52-story steel-framed office building which rises to a height of 623 ft (190m) above the street

level In plan, the tower is typically 190 x 120 ft (58 x 37 m) with set

backs at the 10th, 13th, 23rd, 45th and 47th floors (Fig 1.9a)

The structural system resisting the lateral forces consists of two large vertical flange trusses in the north-south direction and two smaller core braces in the east-west acting as web trusses connecting flange trusses The flange trusses which provide maximum lever arm for resisting the overturning moments, also serve to transfer gravity loads of the core to the exterior columns The resulting equalization

of axial stresses in the truss and the nontruss perimeter columns keeps the differential shortening between them to a minimum To

assure a direct load path for the transfer of gravity load from the core

to the truss columns, the core column is removed below the level of braces at every 12th level, as shown in Fig 1.9b In addition, the step-back corners are cantilevered to maximize the tributary area of gravity load, to compensate for the tensile force due to overturning

Figure 1.8 The Museum Tower,

Los Angeles: Architects: Fuji- kawa Johnson Asso Inc and

- Barton Myers Asso Inc Struc- tural engineers: John A Martin

& Asso Inc., Los Angeles (a)

building elevation; (b) lateral

system; (c) typical post: tensioned floor framing plan

1.3.3 Two Union Square, Seattle

This 50-story office tower (Fig 1.10c) has a curved fagade with widely spaced perimeter columns Lateral resisting elements are placed in the interior core walls enabling the perimeter columns to be spaced

approximately 44 ft (13.42 m) rather than a more typical 10 to 15 ft (3.05 to 4.58 m)

Four 10 ft diameter (3.05 m) steel pipes filled with high-strength, 19,000 psi (131 mPa) concrete are the primary lateral load-resisting elements (Fig 10a,b) To reduce perception of lateral movement in the upper levels of the building, the building’s structural system incorporates 16 dampers Structural design is by Skilling Ward Magnusson Barkshire Inc

1.3.4 Bank of China Tower, Hong Kong

The structural system for the 70-story, 1209 ft (868.5m) Bank of China Tower in Hong Kong consists primarily of a cross-braced space truss The space truss supports almost the entire weight of the building while simultaneously resisting lateral loading of typhoon winds Both the lateral and gravity loads are carried to four

composite columns at the corners of the building, allowing a 170 ft

(51.82 m) clear span at the base of the building °

A fifth composite column in the center of the building begins at the

16 Chapter One

25th floor and extends to the top The loads on this column are transferred to the corner columns at the 25th level At the foundation level, the corner columns are 14 X 26 ft (4.3 X 7.93 m) The size of the steel section of the composite columns varies, and the concrete

portion gets progressively smaller as it rises, varying by more than

10 ft (3.05 m) Compositing of frame elements by enclosing the steel members with reinforced concrete eliminated the need for expensive three-dimensional steel connections at the building corners The structural design is by Leslie Robertson & Associates The structural system is shown schematically in Fig 1.11

1.3.5 Dallas Main Center

The 921 ft (280.7 m) building is of composite construction consisting

of 73-stories of office space A three-dimensional moment-resisting frame made of highly repetitive 36in (0.30 m) rolled shapes spans

CF | = pe steel pipe filled

O 7 O with 19 ksi concrete

Figure 1.10 Two Union Square, Seattle: (a) plan; (b) construction

photograph; (d) building elevation.

127 ft (38.71 m) between columns, giving a height-to-width ratio of the frame of more than 7:1 The structural design is by LeMessurier

Consultants, Inc., Cambridge, Massachusetts

1.3.6 The Miglin-Beitler Tower, Chicago, Illinois

The Miglin-Beitler Tower designed by the New York Office of Thornton-Tomasetti Engineers, if built; will establish a new record

(b)

Figure 1.10 (Continued)

18 Chapter One

as the world’s tallest building as well as the world’s tallest non-guyed

structure surpassing the Sears Tower and the CN Tower which rise

to heights of 1454 and 1822ft (443.2 and 555.4m), respectively

Rising to 1486.5 ft (453m) at the upper sky room level, 1584.5 ft

(483 m) at the top of the mechanical areas, and finally to 1999.9 ft

(609.7 m) at the tip of the spire, the project will provide a regal

landmark to the Chicago skyline An elevation and schematic plan of

the building are shown in Fig 1.18a,b

A cruciform tube structure has been developed to achieve structu-

ral efficiency, superior dynamic behavior, simplicity of construction,

and unobtrusive integration of structure into leased office floor areas

(Fig 1.18f)

Figure 1.10 (Continued)

construction to proceed 8 to 10 floors ahead of the next concrete operation

3 The concrete fin columns, each of which encase a pair of steel

Figure 1.11 The Bank of China Tower, Hong Kong: (a) schematic

elevation; (b-e) floor plans.

20 Chapter One

erection columns, located at the face of the building These fin columns, which extend 20ft (6.10m) beyond the 140 x 140 ft (42.7 X 42.7m) footprint at the base of the building, vary in

dimension from 63X33 ft (2.0X10m) at the base, 53x15 ft

(1.68 X 4.6 m) at the middle, to 43 x 13 ft (1.38 x 4 m) near the top

4 The next components of the cruciform tube system are the link beams which interconnect the four corners of the core to the eight fin columns at every floor These link beams are comprised of reinforced concrete placed simultaneously with floor concrete They become the concrete link between the fin columns and the core to make the full structural width of the building resist lateral forces In addition to the link beams at each floor there are three two story deep outrigger walls located at the 16th story, the 56th